HEAD AND NECK OF THE HORSE 115 



(7) A. ethmoidalis. — The ethmoidal artery is the direct continuation 

 of the ophthalmic, and gains the interior of the cranium by doubling 

 back through the ethmoidal foramen. Its ultimate division into 

 meningeal and nasal branches will be determined later. 



The Third Eyelid (Falpehrsi tertia). — When the eye of a living horse 

 is examined, a prominent semilunar fold of conjunctiva is noticeable 

 medial to the eyeball. This is the membrana nictitans, which covers 

 the thin anterior part of a curved plate of cartilage that forms the basis 

 of the third eyelid. The cartilage is roughly quadrilateral in outline, 

 with a thin, broad, anterior margin, and a narrower and thicker posterior 

 part surrounded by fat and associated with a gland (glandula palpebral 

 tertise) that has a structural resemblance to the lachrymal gland. 

 When the eyeball is retracted, pressure is exerted upon the orbital fat 

 at the posterior part of the cartilage. The cartilage is thus thrust 

 forwards, and the membrana nictitans is protruded over the front of the 

 eyeball. 



Dissection. — The dissection of the eyeball may be made more thorough 

 if several specimens are secured. If eyes of the horse cannot be 

 obtained, those of the ox will serVe the purpose almost equally well. 

 Failing specimens from either the horse or the ox, eyes of the sheep 

 may be used. Some of the specimens are to be examined in the fresh 

 condition : others should be hardened in a solution of formol before 

 dissection. From all the specimens the muscles must be removed. 

 While doing this, notice the exit of the vorticose veins about the 

 equator of the eyeball, and the entrance of the optic nerve and the 

 posterior ciliary arteries. The optic nerve pierces the sclera below and 

 lateral to the posterior pole of the ocular bulb. 



In the first place the dissector should gain some idea of the general 

 structure of the eyeball from sections of at least two specimens. One of 

 these should be cut in a. vertical antero-posterior direction : the other in 

 the plane of the equator. 



The eyeball (Bulbus oculi). — The eyeball is lodged in the anterior 

 part of the orbit, and is protected in front by the eyelids and the 

 conjunctiva. The bony ring formed by the frontal, lachrymal, zygomatic 

 and temporal bones forms a protection for the middle zone of the 

 organ ; while posteriorly it is covered by the periorbita, orbital fascia 

 and fat, and the orbital muscles. 



Although it is customary to describe the eyeball as having the form 

 of an oblate spheroid, this does not give a sufficiently accurate notion of 

 the degree of the characteristic antero-posterior flattening ; nor does it 

 take into account the fact that the cornea, being a segment of a smaller 

 sphere than the sclera, causes a bulging of the front of the eyeball and 

 thus adds to the antero-posterior diameter. In actual figures, the 

 average transverse, vertical and antero-posterior diameters and the 

 distance from the anterior pole to the entrance of the optic nerve may 



